1. Field of Invention
This invention relates to reversible image compression, specifically it suggests the technique for reversible compression of the most problematic part of image: low-order bits.
2. Prior Art
As it is widely used in many reversible formats such as PNG (Portable Network Graphics), TIFF (Tagged Image File Format), JPEG-LS (lossless JPEG) and others, image compression has two sequential operations: data modeling and entropy coding. Data modeling is designed to reduce integers, representing pixel data. Entropy coding is used to replace these integers by shorter binary sequence. Data modeling exploits correlation between rows, columns and colors. For color photographic pictures this correlation is known to be strong in high-order bits and practically not existing in low-order bits, which are treated as an obstruction in compression process. In irreversible compression such as JPEG (Joint Photographic Experts Group) the data modeling is designed to reproduce high-order bits while difference in low-order bits is ignored. This leads to significantly higher compression ratio. Though low-order bits can't be distinguished by the sight of majority of people some business activity require reversible compression of photographic images independently of human perception of quality. The suggested method addresses the problem of compression of low-order bits independently, leaving them out of the scope of compression of remained bits that leads to higher compression.